Correlated subquery in select produces result instead of throwing an error

create table friends (
  id int not null,
  friend_id int,
  primary key (id),
  foreign key (friend_id) references friends (id)
);

insert into friends values
(210, null),
(209, 210),
(202, 209),
(208, 202),
(207, 209),
(203, 207),
(201, null),
(204, null),
(205, null),
(206, 209);

SELECT id,
 (SELECT id
  FROM friends as f2
  WHERE f2.friend_id = f1.friend_id) AS friend
FROM friends as f1
WHERE id = '202'